High molecular weight aromatic polycarbonates and blends thereof with linear polyesters are known, and these can include finely divided particulate fillers having a low aspect ratio, such as silica, and polymeric particles, see, e.g., commonly assigned U.S. patent applications Ser. Nos. 553,172, 553,173, and 553,175, filed Nov. 18, 1983, Richard B. Allen.
Also, it is known to add a foaming agent to thermoplastics as disclosed in U.S. Pat. Nos. 3,781,233, 3,799,954, 4,097,421, 4,097,425, 4,280,005, and 4,351,911.
In U.S. Pat. Nos. 4,280,005 and 4,351,911, it is reported that when a foaming agent is added directly to a polyester, highly irregular foaming occurs which results in an inferior product in that the product contains voids and sink marks. The product is also warped. However, when the foaming agent is added to a polyester containing a filler such as fibrous glass, minerals or mixtures thereof in an amount of at least 5 percent, a foamed product is produced having a rigid cellular core within a solid integral skin. Fibrous glass is a filler with a high aspect ratio, i.e., a length to diameter ratio of greater than 1.0, substantially so.
When a foaming agent is added directly to a polycarbonate, the material will not form a foam when injected into a cavity, the polymer collecting instead as solid masses in the mold. However, it is known to use glass fibers to aid in the formation of gas bubbles which ultimately form cells, and foams. Glass fibers do not, however, produce an entirely uniform, void-free structure with polycarbonates and the glass fibers produce severe and adverse reduction in impact strength, especially at low temperatures and an unsmooth surface appearance, especially if recent techniques, such as counterpressure foam molding, are used to produce the parts. High impact strengths are important because applications of polycarbonate structural foams have begun to expand beyond the traditional business equipment and computer housings into uses such as automobile load floors, seats, truck battery boxes, and the like, and low temperature impact strength, especially with the latter are critical.
It has now been discovered that structural foams comprising aromatic polycarbonate resins and related resins can be nucleated without the presence of glass fiber to yield foams that are superior in impact strength. Nucleating agents that can be used include but are not limited to amorphous silica, precipitated silica, titanium dioxide, particulate impact modifiers, foamed silica, dispersible polytetrafluoroethylene particles, and the like. The use of surfactants is also preferred to help bubble formation and to improve nucleating agent dispersion. Most preferably, the nucleating agent should be sub-micron in size and it should be coated with a hydrophobic coating. Surfactants which can be used include but are not limited to silicones, polyglycol ethers, fluorocarbons, fatty acid derivatives, and the like.
The compositions of the instant invention can be molded, for example, in standard injection molding machines, to produce a variety of foam molded products having good surface appearance, uniform cell structure, and exceptional impact strength.